404 Miscellaneous. 



phenomena always correspond with the centripetal tracheal growths. 

 Shortly after the hatching of the larva a first growth of tracheal 

 tubes makes its appearance towards the mid-intestine. At this 

 moment, at the base of the epithelial cells of that organ, appear the 

 elements of future substitution. A careful study of sections shows 

 that the substitution-cells communicate with the ultimate and very 

 delicate prolongations of the tracheal tubes. They may therefore 

 be regarded as tracheal celis analogous to those seen along the 

 course or at the extremities of the tracheal tubes. 



The elements of substitution are in a state of rest all through 

 the life of the larva ; but from the beginning of nymphosis a 

 renewal of activity sets in: they proliferate actively, join one 

 another and constitute the definite digestive epithelium, whilst 

 the larval tissue enters into histolysis and is thrown off. A fresh 

 tracheal growth appears at this moment ; the calibre and arrange- 

 ment of the respiratory apparatus are modified. At the same time 

 fine tracheoles proceed in great number towards the peri-intestinal 

 muscular layer and penetrate it ; the same occurs in the other 

 muscles of the thorax and abdomen. 



The terminal tracheal cells, or even the cells of the wall of the 

 tracheal trunks, insinuate themselves into the sarcoplasm of the 

 muscular fibres, there multiply actively and form long linear 

 threads ; so that the larval fibre is cut up into little columns, broken 

 up and profoundly altered in form. In the muscular histolysis, 

 whether partial or total, the tracheal cells play an important role 

 by a mechanical process, and probably also chemical, but without 

 the phenomena of phagocytosis. 



Many of the tracheal cells become free in the general cavity and then dis- 

 appear on the spot ; others furnish the tracheoles of the muscles of the imago, 

 these latter turning out the corresponding larval elements (larval muscular 

 fibres and nuclei). 



An American observer, Robert S. Breed*, has described analogous 

 processes in the muscles of a Coieo'^tev {Thymalus). One is inclined 

 to ask with him if it would not be well, in considering the tracheal 

 elements, thus far too much neglected, to again take up the study 

 of the Diptera, in which it is classic to describe an intense phago- 

 cytosis during the metamorphosis. 



In the Hymenoptera the metamorphosis which has just been sketched is 

 completed by the histolysis and total disappearance (veithout phagocytosis) of 

 the primitive Malpighian tubes and the salivary glands, la short, a burst of 

 ectodermic activity realizes the completion of the following organs, momentarily 

 retarded in the larva : teguments, appendages, oesophagus, rectum (formation 

 of fresh Malpighian tubes), nervous system, and sense-organs. 



The tracheal growth is itself a manifestation of this ectodemic 

 activity. It is to be remarked that it corresponds with a period 

 during which Bataillon has noted asphyxial respiratory troubles in 

 Bomhyx mori. — Comptes Rendus, tome cxxxviii. p. 300 (I Feb., 

 1904). 



* R. S. Breed, " The Changes which occur in the Muscles of a Beetle " 

 (Bull. Mus. Comp. Zool. Harvard Coll. vol. xl. no. 7, Oct. 1903). 



